Independent Rear Suspension

Discussion in 'Fiesta ST Chassis Upgrades' started by Ataru, Aug 23, 2013.

  1. Ataru

    Ataru Member

    Since the Fiesta and the FiST have a twist beam rear suspension:


    Do you think a company would come out with an independent rear conversion for it? From the looks of it, something like a Focus suspension could be made for itI, or possibly adapted?

    What are your thoughts?

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  3. Removed

    Removed Guest

    It'd be nice, but it won't come cheap...
  4. wash

    wash Active Member


    In an FWD car, the rear is pretty much just along for the ride. FWD cars are designed with very little weight at the rear also.

    In a rwd car, more weight is back there and keeping both wheels on the ground is critical because they are delivering power. IRS works there with better suspension geometry and reduced unsprung weight compared to a solid axle which is much much heavier than a twist axle that doesn't have to transmit drive torque to the wheels.

    The geometry issue is moot when forces are purely lateral, the total grip is determined by the coefficient of friction (sticktion actually i think) and the normal force which are essentially both fixed. The amount of rubber on the ground has almost no effect and is actually determined by normal force divided by tire air pressure.

    When it comes to unsprung weight, the IRS might have an edge while three wheeling because only 1/2 the unsprung weight is actually moving at that point but it loses because the complexity equals more weight almost every time.

    Lightening the twist axle, adjusting the torsional resistance or adding camber will all have an effect but changing to a custom IRS would be very expensive with questionable benefit.

    No one will do it except AWD conversions like Olsbergs.
    Last edited: Aug 24, 2013
    ST_Rocky and Smokin like this.
  5. Ataru

    Ataru Member

    Why not? How many high end performance cars using a twist beam rear suspension?

    It's just food for thought if a company might try to make or adapt one. Just look at how the rear suspension on the Focus is praised in its ability to help the car rotate in the corners while still maintaining independent movement.

    Anything is possible, just depends on if someone wants to pay for it.

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  6. wash

    wash Active Member

    I thought I covered this:

    It will be expensive.

    It will be heavier than a twist beam guaranteed.

    The only two things you can do with IRS that a twist beam can't is give it a toe curve for "rear steer" in corners and give it a camber curve which is probably no better than static camber when the rear is lightly loaded and body roll isn't huge.

    Everything else is worse except unsprung weight might not be as bad as the extra weight suggests.

    It does almost nothing beside add cost, complexity and weight and there is a cottage industry of suspension kits designed to eliminate "rear steer" because toe changes can feel bad in a corner.

    As for high end performance cars, how many are front wheel drive? I'm going to put that number at zero.

    As Fun as a Fiesta ST is, its a fwd hot hatch. Its not in the same segment but Ford figured out how to make it handle like it is and a light weight twist axle is part of their successful plan.

    IRS on a Fiesta is change for the sake of change unless you are adding AWD and your drive shaft wants to go straight through that twist axle.
    ST_Rocky likes this.
  7. Ataru

    Ataru Member

    I posted this thread looking for people's opinions on the matter, if they would like to see it, would they be willing to spend money for it, not for you to give an answer deemed as the end all.

    This is na discussion forum afterall and I'm looking for multiple people to discuss their thoughts on it.

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  8. reddog99

    reddog99 Active Member

    Umm, I think I probably know what you're getting at, but technically, the twist beam rear end IS independent already. Unlike a pickup with a solid rear axle, each wheel of the Fiesta is free to move independent of the other. The "twist beam" connection between the wheels is nothing more than a built-in anti-sway bar.

    To answer your question, I doubt that there would be enough of a market for the modification you envision for any company to make money at it. And making money at it is the only incentive a company would have, unless they just did it for the fun.
    ST_Rocky likes this.
  9. xorpheous

    xorpheous Member

    +1 Nailed it.
  10. wash

    wash Active Member

    Sorry I don't have an answer you like.

    With regard to technicalities, a twist beam is technically not independent but you are right, it almost perfectly mimics a trailing arm independent with an anti-sway bar and both are a perfectly decent rear suspension for an fwd car.
  11. WRC Fan

    WRC Fan Administrator Staff Member

    I think if small cars become more popular, we may see more of them (not just the MINI) with independent rear suspension. But, cost factoring is always the bugaboo on b-segment cars.

    That being said, while the difference can be felt (on a non-IRS car), I've had no really concerns about my Honda Fit in terms of handling. It's a really fun car to drive, even more fun in some ways than my Abarth. Which also has a twist-beam rear BTW.

    However, while not a "true" anti-roll bar, I did install the Progress Technology rear bar on my FIt early on in its life. I immediately noticed the difference. As it dialed out a bit of the understeer. Not all, of course. But, it did make the car a bit funner to drive on the street. Making it a tad more towards the neutral side. Actually, even introducing some trailing throttle oversteer, if you have road camber on your side. ;)
    ST_Rocky likes this.
  12. reddog99

    reddog99 Active Member

    If it's not independent right now, what would you call it?
    Although maybe not "purely" independent (in some viewpoints), it's certainly a version of an independent suspension.
    Tell me this: if it had been designed as a "pure" trailing arm independent setup from the factory, and you added an anti-sway bar after you bought it home, you'd have what we have right now. But you would call it independent then, but not now?
  13. D1JL

    D1JL Well-Known Member

    I am not trying to get into this argument, however I too believe it acts very much like an IRS.
    In answer to reddog99's question,
    It cannot be called independent as the two sides ARE tied together.
    Therefore it should be called "Interdependent" and in fact, is mutually dependent.

    So, what it comes down to is that the Fiesta does have "IRS" (Interdependent Rear Suspension).

    Just my opinion,
  14. wash

    wash Active Member

    This is a semantic argument but if you take the anti-sway bar off a trailing arm independent rear suspension, you still maintain geometrical control of the wheels, you only lose roll stiffness. If you take the twist beam out of a twist axle rear suspension the wheels will flop around wherever they want.

    In typical analysis of suspension geometry, all of the links are considered to be rigid. Of course if you are designing a twist beam, typical analysis is only the first step and it ends with lots of FEA and prototype characterization because welded joints are very difficult to model with FEA.

    Actually typical analysis of any IRS ignores the anti-sway bar entirely because it has no effect on geometry.

    There is no need to create a new classification of suspension, the twist beam just isn't independent and it doesn't need to be because its virtues out weigh its classification.
  15. D1JL

    D1JL Well-Known Member

    I can agree with all that you have said except.

    That is just wrong because you could/would not build a rear arm on a single mount point.
    It would have to be built so that it could not flop around.
    For example the classic Mini, it uses a longer bolt/axel to prevent the arm from twisting and has no anti-sway bar.
    So assuming the arms did NOT flop around, the geometry would also be unchanged if the beam was removed.
    Then of course you would have independent arms.

    In conclusion the Fiesta's rear suspension is two independent arms tied together with a beam to eliminate additional mounting hardware.
    In addition the interconnecting beam acts as an anti-sway bar.
    This of course makes it an Interdependent Rear Suspension. :p

  16. wash

    wash Active Member

    Take out the twist beam from ^^^ that ^^^ and then explain to me how toe and camber are controlled.

    If you say "its the bushing" you get an F in remedial suspension geometry class.

    In traditional suspension analysis, the arms are broken down in to links, so when you hear about a "five link rear suspension", each side has five links but you can also count an A-arm as two links making two A-arms and a toe link a "five link rear suspension".

    In a trailing arm independent rear suspension the trailing arm is always a two link arm. That constrains the motion in to an arc contained in a set plane.

    A single link only constrains motion to a spherical surface in a 3D space rather than a plane. In layman's terms, it flops around wherever it wants.

    When a twist beam axle still has its twist beam, you can ignore one side and call the twist beam an A-arm with motion constrained to an arc in a plane perpendicular to the line drawn between the two bushings. That's the typical very simplified analysis of a twist beam axle, you ignore half and leave out torsion entirely. If you cut out the connection to the other bushing, now you only have one link, no A-arm and it flops around wherever it wants.

    So what does that tell you?

    Don't cut out the twist beam is a good start.
  17. D1JL

    D1JL Well-Known Member

    Again professor wash, you have proved my statement and the ONLY thing, I have said.

    The Fiesta's rear suspension is two arms tied together with a beam and one cannot work without the other.
    This of course defines Interdependency.

    I truly have enjoyed your lectures.
    I am very glad that you have posted them so I did not need to take notes.
    However you really do read too much into people's statements and then overstate the obvious.
    (more sarcasm) ;)

    Thanks again, but I will have to leave you now.
    I have a life to get back to.

    BTW, getting back to the whole point of this topic.
    No, I don't think an IRS will be built as an aftermarket part for this car, due to cost and very limited potential sales.

    Last edited: Aug 26, 2013
  18. reddog99

    reddog99 Active Member

    No one ever said that it would be great design. The discussion was about whether it was "independent". :p
  19. BRGT350

    BRGT350 Well-Known Member Staff Member

    adding IRS would just add weight and complexity to a simple chassis. I am 100% ok with the twist beam, just the same as I am happy with a well tuned live axle. When I see a twist beam equipped Fiesta R2 sitting on the podium next to Block's MSport Fiesta and a Subaru WRX STi, I figure the twist beam is good enough for anything I will be doing.

    Interesting tidbit of info, did you know the ST has less rear camber than the non ST Fiesta? To get better rotation, the ST beam was machined with less negative camber.
  20. wash

    wash Active Member

    At that point its not even suspension because the wheels would flop out to the side "hella flush" with the chassis resting on the ground.
  21. Gary

    Gary New Member

    I have to laugh a bit when I'm reading about how a IRS would be heavy and costly. I used to have a 1st gen Dodge Neon and it did have a independent rear suspension. It was cheap, simple and worked well. Here is a pic. I also believe that the Honda Civic has one too, another inexpensive car. Now adding a IRS to a car that didn't come with one from the factory, that's another story all together if that is what you guys are talking about. independent rear 1.jpg

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